The dye-sensitized solar cell (DSSC) is a promising alternative to silicon for the creation of cost-effective and flexible solar energy conversion devices. [1,2] DSSC's are constructed by coating a semiconductor film with a layer of light-harvesting dye molecules, to form a dye-sensitized semiconductor. The dye-sensitized semiconductor and a transparent conductive substrate form the photoanode, a second conductive substrate forms the counter-electrode, and the cell is completed using an ion-conductive medium, such as an electrolyte, containing a redox-active mediator, between the photoanode and the counter-electrode. A schematic illustration of a DSSC is shown in FIG. 1, with the transparent conductive substrate shown as SnO coated glass, the semiconductor film is shown as TiO2 particles, and the redox-active mediator is shown as a solution of I− and I3− in ethylene glycol. Also shown in FIG. 1 is a load, electrically connected between the photoanode and counter-electrode, driven by electricity produced by the DSSC upon exposure to light.
The selection of dye and the chemistry of its attachment to the porous semiconductor can dramatically affect both the range of frequencies of light which can be absorbed to generate electron-hole pairs, as well as the efficiency of conversion of the absorbed light into electricity. The current “gold-standard” in DSSC performance is the polypyridyl ruthenium dyes on nanoparticles of titanium dioxide and an I3−/I− electrolyte solution. [3,4] In addition to adjusting the semiconductor surface [5,6] and the electrolyte [2,7] there are considerable research resources being devoted to the molecular engineering of chromophores to improve the cost-efficiency ratio of the DSSC.
N719 and related ruthenium(II) containing sensitizers strongly absorb light with wavelengths below 900 nm. Photoexcitation of these dyes leads to charge transfer from the metal to the bipyridine ligand anchored to the TiO2 surface by the carboxylate group. The existence of the lowest unoccupied molecular orbital (LUMO) in close proximity to the TiO2 surface results in an efficient electron injection into the conductance band for these sensitizers. Furthermore, their high redox potential allows for rapid regeneration of the photo-oxidized dye from an iodide-containing electrolyte. However, ruthenium is a costly resource in limited supply, currently 800 times more expensive than copper (STREM 2008-2010 catalog comparing prices of 99.9% powdered forms).
There have been several reports of DSSCs using copper(I) dyes. [8] While copper-containing dyes have not yet reached the solar efficiencies of the best ruthenium-containing dyes, they also have not been as carefully optimized. The complexes reported previously all contain bipyridines with substituents at the 6- and 6′-positions to stabilize the copper(I) state. They also all contain a carboxylate group either located directly [9,10] on the bipyridine ring or attached via an alkyl [9] or aromatic side-group. [11] There are no metal-containing dyes employing sulfate linkages, although several all-organic dyes containing sulfate linkages [12] have appeared in the literature.